1. Field of the Invention
The present invention relates to a laser-welded steel pipe and a method therefor.
2. Description of the Related Arts
In a conventional method for manufacturing a steel pipe, a steel strip is continuously formed to prepare an open pipe having edge parts, and edge parts facing each other is heated and welded. The steel pipes manufactured by the conventional method are commonly called electroseamed steel pipes, which are widely used in machine structures, pipes in various kinds of plants, line pipes, and parts.
That type of electroseamed steel pipes include general carbon steel pipe, low alloy steel pipe, and stainless steel pipe. Most of the electroseamed steel pipes are, however, occupied by the general carbon steel pipe and the low alloy steel pipe.
Heating of the welding part in the manufacturing method is conducted by high frequency heating or resistance heating. These heating methods have an advantage of high productivity compared with other methods for manufacturing welded steel pipes.
Nevertheless, the electroseam method inherently likely induces fine defects at the welded part, so the method is understood as not applicable for a product required to have high reliability. For instance, electroseamed steel pipes using a stainless steel which contains a large amount of alloying elements are limited in their production and application.
The largest cause of such a limitation is that the electroseam method brings the welding part to an intermediate state between common fusion-welding and pressure-welding, and that the welding part does not form a clear molten pool therein so that the inclusions such as oxides formed during welding process are difficult to be discharged from inside of the steel structure.
To cope with the disadvantage of electroseam method, various technologies have been introduced. Among them, a method for improving the disadvantage caused by incomplete fusion of the above-described welding part was disclosed in unexamined Japanese patent publication No.56-168981. According to the disclosure, the edge parts facing each other on an open pipe are pre-heated by high frequency heating followed by melting the junction point of edge parts with laser beam at near the squeeze roll to conduct pressure-welding.
In that manner, a combination of conventional electroseam method with laser heating method ensures the edge parts to finally fuse to weld together. Accordingly, the defects caused by inclusions which are a problem of electroseam method are expected to significantly reduce. In addition, pre-heating suppresses incomplete penetration which often occurs in a high speed laser-welding.
The above-described technology disclosed in unexamined Japanese patent publication No.56-168981 may be said to be a basic technology for manufacturing steel pipes at a high speed without generating defects. For applying the technology to a commercial production line, however, there are many issues to be solved. The largest issue of them is to determine the quantity of irradiating energy to satisfy the welding.
Laser-welding has also a colliding relation between the increase in welding speed (increase of productivity) and the defect rate. That is, increase of irradiating energy quantity or decrease of pipe manufacturing speed increases the probability of removal of generated inclusions from the molten steel pool, thus providing a welded part having less inclusions. However, the condition decreases the productivity.
There is a similar relation on the tendency of gas blow holes. That is, bubbles formed during melting process float up through the molten pool as observed in the behavior of inclusions, and finally the bubbles go out from the pool. If the laser irradiation energy is strong, the cooling speed of the steel becomes slow, and sufficient time is expected to ensure until considerable amount of bubbles float up to the surface of pool. To the contrary, if the laser irradiation energy quantity is less, the molten pool rapidly solidifies, thus the steel solidifies before sufficient number of gas bubbles escape from the steel, and finally a welded part containing lots of blow holes appears.
The laser irradiation heating is a method to concentrate energy onto a narrow range. Accordingly, in a steel pipe manufacturing process, a high temperature molten pool and a steel at near normal temperature are located closely to each other. As a result, the molten pool which was formed by the laser irradiation is rapidly cooled by transferring the heat to the surrounding low temperature steel. Consequently, even when the laser irradiation increases the heat input, the above-described defects likely remain. In addition, strain at near the welded part is large, and the residual stress is also large. Furthermore, the laser-welding does not necessarily transfer a large quantity of energy compared with the consumed electric power, and a laser generator with large output is expensive.
The technology disclosed in unexamined Japanese patent publication No.56-168981 which is described above adopts pre-heating before laser irradiation to solve the above-described problems of laser-welding, or the problems of energy cost and of rapid cooling. The pre-heating by high frequency heating has a function to heat a broad area at near the edge parts of open pipe compared with laser irradiation. The heating method provides high heating efficiency per input energy.
Welded pipes are requested to assure the defectless condition and to be produced in good efficiency and at a low cost. That is, the optimum pre-heating condition and the optimum laser heating condition should be established while assuring the characteristics of the steel pipe and satisfying the economy. Accordingly, if the optimization of economy (productivity) comes first, and thus increasing the input of heat by pre-heating and minimizing the input of heat by laser irradiation to let the melted part exist simply by the laser irradiation, then no satisfactory steel pipe is obtained.
For example, when the above-described pre-heating is conducted in air, oxides deposit on the surface of the steel, and oxides also deposit on the edge parts facing each other on the open pipe. The edge parts melt under the laser irradiation, and the oxygen concentration in the steel increases. The hot-rolled steel strip which forms the open pipe is prepared by a steel that was fully deoxidized. During the welding operation, passage of laser beam at near the welding part is shielded by argon or other inert gas, so the amount of atmospheric components (particularly oxygen) which are brought into the molten steel pool during the welding process is not so large.
Chemical reactions occurred in the molten pool which was formed by laser irradiation is discussed below.
As described before, oxide film generated during the pre-heating in air exists on the steel surface. When the steel fuses, carbon in the steel and oxygen in the oxide film react to yield CO and CO.sub.2 gases to induce bubbling. The bubbling then induces the reactions for generating spatters and gas bubbles, which results in blow hole defects. Diminishing carbon from steel results in the reduction of steel strength, which is also a problem.
To prevent the reactions yielding CO and CO.sub.2 gases, either one of O or C should be prevented from entering the molten pool. Even when the amount of O in the molten pool is large, less amount of C makes the reaction for yielding CO and CO.sub.2 gases difficult to proceed.
Carbon is, however, a most common element to ensure the strength of steel. Therefore, simple reduction of the amount of C is not acceptable, and the above-described measures are not applicable. As a result, reduction of oxygen content becomes an alternative measure. There is, however, no established method to reduce the amount of oxygen on commercial basis. Thus, the present state is that, even for a welded steel pipe produced by minimizing inclusions by laser irradiation, the presence of blow hole defects is accepted as unavoidable.